Conventional electrophotographic apparatus, such as copiers, printers, facsimile machines, etc., comprise an imaging surface, such as a photoconductive element, normally in the form of a drum or belt. Arranged in timed sequence around the drum are a plurality of processing stations for performing various functions. These processing stations may comprise apparatus for charging the imaging surface, electrostatically forming a latent image on the imaging surface, developing the latent electrostatic image with a developer commonly referred to as a toner, transferring the developed image from the imaging surface to a substrate such as a paper sheet, typically by means of a transfer roller, feeding paper to the transferring station, cleaning the imaging surface, i.e., removing residual toner on the imaging surface, and fixing the transferred developed image on the sheet.
A typical reproduction operation comprises charging the surface of the drum and exposing the charged surface to a light pattern of an original image to be reproduced thereby selectively discharging the imaging surface in accordance with the original image. The resulting pattern of charged and discharged areas on the surface of the photoconductive drum forms an electrostatic charge pattern or electrostatic latent image conforming to the original image.
The latent electrostatic image is developed by contacting it with finely divided toner held by electrostatic force on the imaging surface. The toner image is transferred to the sheet fed by a registration roller toward the drum in synchronization with drum rotation. As the leading edge of the sheet abuts the drum, electrostatic forces adhere the two together, and the transferring station applies toner image from the photoconductive drum to the paper. Thereafter, the toner image is fixed to form a permanent record.
Subsequent to development, and after transfer of the developed image to the paper, some toner inevitably remains on the surface of the photoconductive drum, held thereto by electrostatic and/or VANderWals force. Additionally, other contaminants, such as paper fibers, toner additives, Kaolins and various other forms of debris, have a tendency to be attracted to the charge retentive surface. This residual toner has a tendency to contaminate the backside of a sheet of paper whose front surface is to receive the developed toner image, despite conventional roller and drum cleaning techniques.
Referring to FIGS. 11a-11d, depicted schematically is an image forming apparatus comprising a photoconductive drum 1 that rotates in direction A such that the surface of the drum successively traverses contact charging roller 2, developing roller 4, transfer roller 5, cleaning blade 7 and pre-discharging lamp 8. The transfer roller 5, which fixes the toner image to a sheet fed to the drum 1 between developing roller 4 and transfer roller 5, is generally formed of a conductive Styrofoam having a porous surface. Toner remaining on the surface of drum 1 from developing roller 4 tends to deposit onto the surface of the transfer roller and into its pores. As the transfer roller 5 contacts the photoconductive drum 1 in a contact area establishing a nip, the toner remaining on the transfer roller tends to accumulate on the backside of the sheet, undesirably producing a darkened region. It has been proposed to clean the surface of transfer roller 5 using a cleaning blade; however, this has not proved to be practical because the blade has a tendency to push the toner on the surface of the roller into the roller pores, rather than remove the toner completely.
In order to clean the transfer roller 5 more effectively, it has been proposed to apply a bias voltage to the transfer roller of appropriate polarity and magnitude so as to electrostatically repel residual toner from the transfer roller 5 to the surface of the photoconductive drum 1 for subsequent removal. See, for example, U.S. Pat. No. 5,253,022, based on the realization that most toner has a negative polarity and some toner has a positive polarity, as depicted symbolically in application FIG. 11a. At first, a negative polarity bias is applied to the transfer roller 5 for a prescribed period of time, and then a positive bias is applied for a shorter time, as shown in FIG. 11b. Positive and negative polarity toner are expelled from the transfer roller 5 to the surface of the photoconductive drum 1, to be removed from the drum by cleaning blade 7. However, the surface of drum 5 now carries regions of positively and negatively induced electrostatic charge corresponding to the applied transfer roller bias.
FIG. 11c depicts removal of the residual toner from the surface of drum 1 by cleaning blade 7. The region of the drum surface 1 between cleaning blade 7 and charging roller 2 is irradiated by the pre-discharging lamp 8 to discharge negative charge on the drum surface (FIG. 11d). However, the pre-discharging lamp 8 has no effect on positively induced charge, which will remain on the surface of the drum 1, as shown symbolically. As the drum continues to rotate, the positive surface charge traverses the developing roller 4, picking up negative polarity toner which deposits onto the back side of the next sheet waiting for image development and fixing.